O Level Chemistry Chap 14: Metals

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Metals have a regular arrangement of closely packed positive ions surrounded by “a sea of mobile valence electrons”. Atoms in a metal are packed tightly in layers and held by strong metallic bonds.

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Properties of metals and the relation to its structure Physical Property Relation to structure Malleable (easily beaten into thin Atoms (of the same size) are sheets) and ductile (can be drawn regularly arranged in layers. They can into wires without breaking) slide over each other easily when forced is applied. High density Metal atoms are closely packed. High melting and boiling points Metal atoms are packed tightly in layers by strong metallic bonds. A lot of energy is needed to break the strong metallic bonds. Good conductors of heat and In metals, the valence electrons of electricity the atoms are mobile and can move throughout the metal. They allow heat and electricity to be conducted.

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An alloy is a mixture of a metal with one or a few other elements.  Bronze = Copper + Tin  Brass = Copper + Zinc (Musical Instruments, decorative ornaments)  Stainless steel = Iron + Chromium + Nickel + Carbon (Cutlery, surgical instruments)

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Why metals are often used in the form of alloys a) Alloys are harder and stronger than pure metals. Structures of Metals VS Alloys Metals Alloys Pure metals are weak and soft. The atoms - When a pure metal is alloyed, a different (of the same size) are regularly arranged element is added to the pure metal. in layers which can slide over each other - This disrupts the regular arrangement of easily when force is applied. Pure metals atoms in the pure metal by the addition of are thus soft and malleable. other atoms of a different size. - Therefore, it is difficult for the atoms in an alloy to slide over each other. - This makes alloys harder, stronger and less malleable than pure metals.

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[Chemistry] Chap14: Metals

Metals and Alloys

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b)

While pure metals corrode easily, alloys have a higher resistance to corrosion. E.g. Copper-nickel alloy is used to make coins instead of pure copper.

c)

Alloying is used to lower the melting points of metals. E.g. Solder is an alloy of tin and lead and has a lower melting point than its constituent metals, thus can be used to join metals.

The Reactivity Series 4. Chemical Properties of Metals:  Form positive ions (cations) by loss of electrons (metals are reducing agents)  Form ionic compounds e.g. metal chlorides/oxides  Usually (but not always) react with dilute HCl or H2SO4 to produce salt + hydrogen  React with oxygen to form basic oxides or amphoteric oxides Reactivity Series  The reactivity series is a measure of a metal’s tendency to lose electrons and form a positive ion. Metals high up in the reactivity series have a greater tendency to form its positive ions.  The order of reactivity can be deduced by reference to - the metal’s reaction with water, steam and dilute HCl - the reduction of their oxides by carbon and/or hydrogen, or any other form of decomposition.  Reactive metals are unstable and tend to react to form compounds (compounds are stable). Unreactive metals are more stable but their compounds tend to be less stable than those of the more reactive metals.

[Chemistry] Chap14: Metals

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Reactivity Series of 13 metals Reactivity

Element

Extraction Carbon Hydrogen Heat (oxide) (oxide) (carb) x x x x x x x x / x x / x x / / x / / / / / / /

Heat oxide Electrolysis with coke

Heating or physical extraction

Water

Steam

Dilute HCl/H2SO4

Potassium Sodium Calcium Magnesium Aluminium Zinc Iron Lead Hydrogen Copper / / / Manganese Heat Heat Silver Heat Heat /* Gold Heat Heat Platinum Heat Heat Extraction method is from the metal oxide for Carbon and Hydrogen, and the metal carbonate for Heating. Refer to TB for details on the reactions of metals. Note: - Metals above hydrogen will react with dilute acids to produce hydrogen gas (they are more reactive and hence can displace hydrogen from solutions of acids). - Lead appears not to react with dilute HCl/H2SO4 which is an exception to the acid + metal reaction (refer to chap on acids and bases). However, just remember that metals above hydrogen will react with dilute acids. - Dilute H2SO4 reacts with metals similarly to HCl, the salt formed is a metal sulfate. -

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K to Ca reacts with steam too, but is similar to reacting with water. There is some ambiguity at the borderlines between the groups. Mg is considered not to react with water (although it can) as the reaction is too slow unless the surface layer of oxide protecting the metal is specially removed. Al appears relatively unreactive despite its high position in the reactivity series. This is because Al is reactive and reacts with oxygen, forming a thin and protective layer of Al2O3 which coats the metal and prevents further reaction. This can be used to prevent corrosion (see Alloys)

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Reduction of Metal Oxides by Reducing Agents  The reactivity of metals can be studied by how easily metal oxides decompose. The more reactive a metal is, the more difficult it is to decompose its oxides (reduce the oxide to the metal)  Above zinc = not reduced by reducing agents (too stable, use passing electricity).  Below copper = oxide will decompose simply by heating without needing a reducing agent. 2Ag2O (s)

4Ag (s) + O2 (g)

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[Chemistry] Chap14: Metals

Applications of the Reactivity Series

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a) with Carbon (Zn & below) [smelting]  

E.g. 2CuO (s) + C (s) Cu (s) + CO2 (g) Cu2+ ions from copper (II) oxide are reduced to copper; carbon is oxidised to carbon dioxide. Metals below magnesium are often extracted from their ores (industrially) by reduction with carbon. [?!]

b) with Hydrogen (Fe & below)  metal oxide + hydrogen metal + steam  CuO (s) + H2 (g) Cu (s) + H2O (g) 7.

Decomposition of Metal Carbonates  Compounds of reactive metals are stable and not easily reduced and decomposed by heat (more stable to heat/thermally stable). The more reactive a metal is, the more difficult it is to decompose its compounds (e.g. it takes a lot of heat and time to decompose CaCO3, but CuCO3 decomposes quickly and easily).  Carbonates- Ca & below: 

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metal carbonate metal oxide + carbon dioxide Silver carbonate: the silver oxide produced further decomposes to form silver + oxygen (unstable) 2Ag2CO3  4Ag + 2CO2 + O2

Displacement of Metals  More reactive metals can displace a less reactive metal from its salt solution or oxide. This is because more reactive metals have a greater tendency to form its positive ions. E.g. Mg can displace ZnSO4 but Cu, Pb and Fe cannot.  The more reactive a metal is, the more readily it forms compounds; unreactive metals tend to stay uncombined.

b) from oxide  More reactive metals can reduce the oxide of a less reactive metal.  E.g. Thermite reaction- Al displaces Fe from Fe2O3 Fe2O3 (s) + 2Al (s)  Al2O3 (s) + 2Fe (l) Al atoms react with O2- ions from Fe2O3 to form Al2O3, Fe2O3 is reduced to molten Fe.  Note: Highly exothermic reaction, molten iron (which melted due to heat) is formed. This reaction is used to weld railway lines together. 9.

Using the Reactivity Series Refer to TB/242

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[Chemistry] Chap14: Metals

a) from solution  E.g. iron displaces copper ions in CuSO4 to form copper metal and FeSO4 Fe (s) + CuSO4 (aq)  FeSo4 (aq) + Cu (s)  Metal displacement reactions are redox reactions. The more reactive metal is oxidised (by becoming ions and forming compounds) while the less reactive metal is reduced (by changing back to atoms). E.g. Fe (s) + Cu2+ (aq)  Fe2+ (aq) + Cu (s) Copper ions are reduced to copper atoms; iron is oxidised to iron (II) ions.

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Extraction of Metals 10. Metals are found in the ground in rocks called ores. E.g. iron is found in the ore haematite. An ore is a compound of the metal (oxides, sulphides, chlorides or carbonates) mixed with large amounts of earth and rock. 11.

Metal K to Al Zn to Pb Cu to Pt

  

Method of Extraction From ore: Using electricity to decompose the molten metal compounds (electrolysis) From metal oxide: Reduce the metal oxide by heating with coke (smelting) Found free in the ground naturally as uncombined metals / physical extraction

The more reactive the metal is, the harder it is to extract the metal from its ore. E.g. compounds of reactive metals are stable and difficult to be reduced. Whenever possible, reduction with coke is used. Electrolysis is only used for reactive metals because carbon cannot take oxygen away from the metal oxide, as the bonds in the metal oxide are too strong. Smelting is cheap while electrolysis is expensive.

Extraction of Iron from Haematite 12. Refer to TB/244-246  The iron that is extracted from the blast furnace is known as cast iron, and is mainly used to produce steel.

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Steel is an alloy of iron with carbon and/or other metals. Different types of steel are made by varying the amount of carbon and by adding different metals to iron. Each type has different amounts of carbon and other metals added, hence its own unique properties and uses. Carbon steels: Iron + Carbon Alloy steels: Iron + Carbon + One or more of: Mn, Ni, Cr, W, V. These metals are added to change the properties of the steel.

Category Carbon Steels

Type of Steel Mild Steel (low carbon) 0.25% Carbon High Carbon Steel 0.45 – 1.5% Carbon

Alloy Steels

Stainless Steel Alloy of iron, chromium, nickel & carbon.

Uses Car bodies and machinery Cutting and boring tools, e.g. knives, hammers Equipments in chemical plants, cutlery, surgical instruments

Special Properties Hard, strong and malleable Strong but brittle (more carbon atoms to prevent sliding) Extremely durable, resistant to rust and corrosion even when heated

Qn: Explain how the properties of low carbon and high carbon steel differ. Low carbon steel is softer as it is more malleable. High carbon steel contains more carbon atoms which prevent sliding of the iron atoms. Hence, high carbon steel is harder but brittle.

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[Chemistry] Chap14: Metals

Steel 13.  

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Rusting 15.    

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Rusting is the oxidation of iron to form hydrated iron (III) oxide. When an object made of iron (e.g. iron, mild steel) is exposed to moist air for some time, a reddish-brown substance (rust) slowly forms on the surface, through rusting/ corrosion of iron. Rusting occurs only in the presence of both oxygen and water. The presence of sodium chloride and acidic substances (e.g. sulphur dioxide, CO2) speeds up the rusting process. A simplified equation is iron + oxygen + water  hydrated iron (III) oxide [rust] 4Fe (s) + 3O2 (g) + 2xH2O  2Fe2O3.xH2O (s) Note: - Mild steel will rust faster than high carbon steel due to higher iron composition - Only iron (and alloys containing iron) will rust. Magnesium corrodes.

Rust Prevention Method Using a protective layer

Using a sacrificial metal (sacrificial protection)

[Chemistry] Chap14: Metals

Using alloys

Description - The metal may be coated with a layer of substance that stops water and air from reaching the metal. - This is done by painting, greasing, and coating with another metal. - E.g. In galvanising (or zinc-plating), a thin film of zinc is used to cover the iron. Note: even if the zinc layer is damaged, the iron will not rust because zinc is more reactive than iron and it will corrode in place of iron (the more reactive metal corrodes preferentially). - Involves attaching a more reactive metal (e.g. zinc) to iron. Since zinc is more reactive than iron, it gets corroded in place of the iron because zinc loses its valence electrons more readily. As long as a more reactive metal is present, iron will not rust. - E.g. fixing bars of zinc to a ship’s hull prevents the ship’s steel body from rusting, attaching magnesium blocks to underground pipes - Stainless steel, which is a rust resistant alloy, can be used. On exposure to air and moisture, a very hard coating of chromium (III) oxide forms on the surface of stainless steel, preventing it from further corrosion.

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Recycling of metals 17.  Metals are finite resources and need to be conserved.  The amount of metal ores in the Earth is limited. If metal extraction continues at present rates, the supplies of many metals will run out. Hence, there is a need to recycle metals.

Economic

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Social

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Environmental

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Advantages It saves the costs of extracting new metals from their ores. Fewer landfills to dispose used metal objects will need to be built. This saves the cost of building landfill sites. It helps to conserve the limited amount of metals in the Earth. With the increasing world population, more land will be available if ore mining is reduced. Reduces the environmental problems related to extracting metals from ores (e.g. waste gases including carbon monoxide which are formed in the blast furnace through iron extraction will be reduced)

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Disadvantages It is very expensive, considering other processing costs like collecting, transporting and separating the scrap metals.

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It takes time and effort for the world to practise recycling as a way of life.

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The recycling process may cause pollution if not done properly (e.g. metal fumes from the recycling process)

Q1

Some alloys are coated with a layer of aluminium. Explain how this prevents the alloy from corroding. Al, being a reactive metal, reacts with oxygen, forming a thin and protective layer of aluminium oxide which coats the alloy and protects it from further corrosion.

Q2

Explain why magnesium blocks are often attached to underground steel pipes. Magnesium, being more reactive than iron, acts as a sacrificial metal to corrode in place of iron. It provides sacrificial protection against rusting for the steel pipes.

Q3

A block of iron is coated with copper. The block was accidentally scratched and the iron beneath was exposed. a) Explain what would happen to the iron. b) Will the copper in contact with the iron corrode faster or slower than normal? Explain your answer. a) The iron will undergo rusting because it is now exposed to oxygen and water (moisture in air). In addition, the iron will rust faster than normal. This is because iron, being more reactive than copper, acts as a sacrificial metal to corrode in place of iron (when itself already undergoes rusting). b) The copper will corrode slower than normal, because iron provides sacrificial protection for the copper against rusting by corroding in place of copper.

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[Chemistry] Chap14: Metals

Sample Questions

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The Reactivity Series Metal K Potassium Na Sodium Li Lithium Sr Strontium Ca Calcium Mg Magnesium Al Aluminum Carbon Zn Zinc Cr Chromium Fe Iron Cd Cadmium Co Cobalt Ni Nickel Sn Tin Pb Lead Hydrogen Cu Copper Ag Silver Hg Mercury Au Gold Pt Platinum

Reactivity React with water

Extraction

Electrolysis

included for comparison

React with acids

Smelting with coke

included for comparison Found naturally Highly unreactive uncombined Heat or physical extraction

Going from bottom to top, the metals:     

increase in reactivity; lose electrons more readily to form positive ions; corrode or tarnish more readily; require more energy (and different methods) to be separated from their ores; become stronger reducing agents.

Notes:

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[Chemistry] Chap14: Metals

Carbon is able to reduce metal oxides Zn to Pb because it is “higher up” in the reactivity series than them.

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